Ar's are a great home defense tool, much better than a pistol by far... shooting 55gr hp bullets cut over penetration out completely... there are a lot of great ar's out there one that seems to slip under the radar constantly is bushmaster.
as far as S&W's avoid them right now, i have three of them in class right now that are all brand new less than 100-200rds fired that have loose barrels and barrel nut/delta rings. Its a problem that will probably be resolved quickly, amazing what happens what a gunsmithing college calls a company and says wtf... but for the time being avoid them
also look at the price difference between the s&w and the stag... they are the same guns with a different roll stamp.
The biggest point I can make is TRAINING TRAINING TRAINING!!!
Any gun is absolutely useless without good training